We would suggest using a long handled scrub brush (a boat brush works very well), and a bucket of warm water, detergent and 25 percent bleach. Some mild scrubbing, with the grain, will be required. In areas with lots of mildew, you might increase the bleach content. Use a hose to rinse the area liberally. After the deck has completely dried, apply a wood sealer. Do not use a general-purpose sealer designed for wood, concrete, masonry, etc. Use one specifically designed for wood, which contains UV protection, and mildew inhibitors. Special applicators are now made for applying wood sealers. They look like a wide sponge mop, and come with a special tray. They work quite well. The only downside of using sealers such as this, is that the deck stays wet for significantly longer after a rain. While manufacturers often claimed that the wood sealer will last two years, our experience suggests an annual application is required.